Before insulating window frames, I would consider the possibility that you may have too much moisture in the air. The human body on average releases 5 cups of moisture into the air per day and cooking and showering releases even more moisture into the air. You need to crack open vents, windows and/or if you have one, run a Fantastic fan on low to get a bit of air drawn out. In cold weather you should't seal up an RV and make it airtight in an effort to retain as much heat as possible. Another way of reducing moisture is to use a dehumidifier and then you can open a window/vent just enough to exhaust odors & stale air (CO2). Having a hygrometer might be a good idea so you tell exactly what the humidity level is.
Moisture can also condense on other surfaces like aluminum framing & roof trusses and at the worst, on underside of the roof deck and exterior wall skin. RVs don't have a vapor barrier on the warm side of walls and ceilings like in a house and moisture will migrate through insulation to cold surfaces. This possibly shouldn't be ignored because it can eventually lead to serious damage.